An inline electron gun is one designed to generate or initiate preferably three electron beams in a common plane, and to direct those beams along convergent paths in that plane to a point or small area of convergence near the tube screen. In one type of inline electron gun, such as that shown in U.S. Pat. No. 3,873,879, issued to R. H. Hughes on Mar. 25, 1975, the main electrostatic focusing lenses for focusing the electron beams are formed between two electrodes referred to as the first and second accelerating and focusing electrodes.
The concept of utilizing two electrostatic focusing lenses to form an effective larger main focus lens is disclosed in U.S. Pat. No. 2,975,315, issued to C. S. Szegho on Mar. 14, 1961; in U.S. Pat. No. 3,852,637, issued to E. Yamazaki et al. on Dec. 3, 1974; and in U.S. Pat. No. 4,334,169, issued to S. Takenaka et al. on June 8, 1982. In each of these patents, four electrodes are used to form the two electrostatic focusing lenses. In each patent, one lens is formed by three of the electrodes, with the center electrode being excited with a lower voltage than the two-side electrodes which are electrically connected. The other lens in these patents is formed by two electrodes excited with different voltages.
An inline electron gun wherein a bipotential electrostatic focusing lens is expanded in size is disclosed in U.S. Pat. No. 4,370,592, issued to R. H. Hughes et al. on Jan. 25, 1983. In this patent, the enlarged lens is formed by setting back or recessing the three inline apertures in each of two focus electrodes so that the rims around the recesses which face each other provide the primary control in forming the main focus lens.
In an earlier inline electron gun, described in U.S. Pat. No. 3,448,316, issued to S. Yoshida et al. on June 3, 1969, three electron beams pass through a first lens, created by two electrodes, each having a single large aperture, which causes the two outer electron beams to cross-over at a second lens, formed by three electrodes, each having a single large aperture. The electron beams diverge after leaving the second lens and pass between convergence plates which deflect the electron beams to cause them to converge near the tube screen.
The above-described electron guns were originally developed for broadcast-type television picture tubes. However, more recent development of data display tubes has mandated further improvement in electron guns to provide a smaller, sharper electron beam spot at the tube screen. The present invention meets this need for an improved gun which can provide higher resolution images on a display screen.